老生常談，循環依賴！顧名思義嘛，就是你依賴我，我依賴你，然後就造成了循環依賴了！由于A中注入B，B中注入A導緻的嗎？
看起來沒毛病，然而，卻沒有說清楚問題！甚至會讓你覺得你是不清楚spring的循環依賴的！
那麼，spring的循環依賴到底是啥玩意？
來看個循環依賴注入失敗的例子：
bean1: userService
@Service
public class LoginServiceImpl implements LoginService {
// @Resource
private UserService userService;
public LoginServiceImpl() {
// 無參構造器是給預設建立的bean使用的
}
@Autowired
public LoginServiceImpl(UserService userService) {
this.userService = userService;
}
public void setUserService(UserService userService) {
this.userService = userService;
}
public Integer login(UserInfo userInfo) {
System.out.println("login...");
return 0;
}
public void logAction(String userId, String action) throws Exception {
UserInfo userInfo = userService.getUser(userId);
System.out.println("logged..." + userInfo + ",action: " + action);
}
} bean2: loginService
@Service
public class UserServiceImpl implements UserService {
// @Resource
private LoginService loginService;
public UserServiceImpl() {
// 無參構造器是給預設建立的bean使用的
}
// @Autowired
public UserServiceImpl(LoginService loginService) {
this.loginService = loginService;
}
@Autowired
public void setLoginService(LoginService loginService) {
this.loginService = loginService;
}
public Integer addUser(UserInfo userInfo) throws Exception {
userInfo.setId("111");
System.out.println("user added. " + userInfo);
// 登入
loginService.login(userInfo);
return 1;
}
public UserInfo getUser(String id) throws Exception {
UserInfo userInfo = null;
if("111".equals(id)) {
userInfo = new UserInfo();
userInfo.setId(id);
userInfo.setAge(22);
userInfo.setName("mocking");
return userInfo;
}
throw new Exception("err:" + ErrorCodeEnum.API_CALL_ERROR.getCode() + "," + ErrorCodeEnum.API_CALL_ERROR.getErrMsg());
}
} 然後再建一個測試類，來測試以上代碼！
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath:applicationContext.xml"})
public class UserServiceTest {
@Resource
private UserService userService;
@Test
public void testAddUserForLoopDI() throws Exception {
UserInfo userInfo = new UserInfo();
userInfo.setId("12");
userInfo.setAddress("sdfds");
userService.addUser(userInfo);
System.out.println("ok");
}
} applicationContext.xml 中隻要開啟掃描即可！
<context:annotation-config/>
<context:component-scan base-package="com.xx"></context:component-scan> 如上例子運作，就可以得到一個循環依賴的失敗錯誤了！
不過，你不一定能運作進來，因為如例子還要依賴于一個事實，那就 loginService 要在 userService 之前被掃描到，而不同的jvm上，可能spring得到的掃描順序不一緻，如果想要100%失敗，則換成兩個類都是構造器注入就可以了！
錯誤堆棧樣例如下：（Is there an unresolvable circular reference?）
java.lang.IllegalStateException: Failed to load ApplicationContext
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)
at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:83)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:117)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:83)
at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:230)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:228)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:287)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:289)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:247)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'loginService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'userService' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)
at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1198)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1100)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:128)
at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:60)
at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.delegateLoading(AbstractDelegatingSmartContextLoader.java:281)
at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.loadContext(AbstractDelegatingSmartContextLoader.java:249)
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:98)
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:116)
... 24 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1534)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1281)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:551)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
... 44 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
... 54 more
java.lang.IllegalStateException: Failed to load ApplicationContext
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)
at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:83)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:117)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:83)
at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:230)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:228)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:287)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:289)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:247)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'loginService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'userService' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)
at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1198)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1100)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:128)
at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:60)
at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.delegateLoading(AbstractDelegatingSmartContextLoader.java:281)
at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.loadContext(AbstractDelegatingSmartContextLoader.java:249)
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:98)
at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:116)
... 24 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1534)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1281)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:551)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
... 44 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
... 54 more View Code
好了，看完效果，是時候來分析下問題了。
1. 為什麼會出現問題？
因為加載流程如下：
1. 首先加載 loginService, 然後構造器被調用，然後發現依賴了 userService;
2. 依賴注入，先去加載 userService，加載userService後，需要對其屬性進行依賴注入，然後發現了 loginService 需要被注入；
3. 當去加載 loginService 的時候，發現 loginService 正在建立中，是以這個執行個體隻能認為建立失敗了，否則将會導緻更多未知問題；
4. 同理，多個構造器互相注入失敗問題更嚴重；
2. 循環依賴失敗是不是隻要 ABA 就一定會導緻失敗？（注： 非單例對象一定不會導緻循環依賴）
1. 按正常說是這樣的，但是spring已經解決這個問題了；
2. spring 解決方案為，隻為單例提供解決方案也隻能為單例解決問題；
3. 建立A單例時，放入緩存，然後依賴注入B；
4. 依賴注入B時，發現需要依賴注入A，然後去加載A，此時從緩存中發現A正在加載中，于是直接從緩存得到A，完成自身的依賴注入；
5. B依賴完成後，傳回給A，A再把B注入到自身域中；
6. B中的A也自然而然的完成了初始化動作；
其中，構造器注入的單例的循環依賴是無法解決的，因為在構造器注入時，本身的執行個體無法生成；如果強行使用，将導緻不安全的釋出，進而導緻各種未知的問題！
3. 源碼如何實作？
1. 首先，我們來看下什麼情況下會抛出循環依賴異常？
// 隻有單例的建立會存在循環依賴問題
// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry
/** Set of registered singletons, containing the bean names in registration order */
private final Set<String> registeredSingletons = new LinkedHashSet<String>(256);
/** Names of beans that are currently in creation */
private final Set<String> singletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));
/** Names of beans currently excluded from in creation checks */
private final Set<String> inCreationCheckExclusions =
Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
從上面的代碼可以看出，抛出循環依賴問題會抛出有兩種前提:
1. 沒有包含在建立檢查的排除清單中；
2. 沒有被排除，則檢查是不是第一次被調用建立，如果單例不是第一次被調用建立，則不能再建立了（否則就不是單例了）；
是以，避免循環依賴有個出口，那就是提前把單例放到檢查排除清單中！
那麼什麼時候會被加入到建立檢查排除清單？
public void setCurrentlyInCreation(String beanName, boolean inCreation) {
Assert.notNull(beanName, "Bean name must not be null");
if (!inCreation) {
this.inCreationCheckExclusions.add(beanName);
}
else {
this.inCreationCheckExclusions.remove(beanName);
}
}
具體如何設定呢？其實普通的使用不會用到這個功能，隻會在一些增強點作這些工作。
是以，spring 如何避免循環依賴失敗？
// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean()
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// 第一步，建立bean執行個體
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// 對于提前暴露的bean, 将它加入單例緩存中，進而使後續的依賴調用時，可以将此執行個體傳回
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// 針對提前暴露的 singleton, 添加一個 擷取執行個體 工廠，以備後續擷取執行個體時使用
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// 第二步，初始化執行個體，完成依賴注入等工作
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// 第三步，對外暴露bean，如 singleton 入 容器中注冊執行個體
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
} 而在擷取 singleton 時，會先嘗試從工廠中擷取！
// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
// 針對正在建立中的 singleton, 将其放入 earlySingletonObjects
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
// 将 singleton 執行個體從工廠中取出後，存入 提前暴露的緩存中，交将建立工廠删除，避免後續反複從中擷取 singleton
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.getEarlyBeanReference()
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
if (exposedObject == null) {
return null;
}
}
}
}
return exposedObject;
} 在擷取bean執行個體時，會先嘗試從緩存中直接擷取，失敗再進行真實的建立：
// org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean()
// 會先嘗試從單例緩存中擷取bean，擷取不到再進行建立
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// 先從緩存擷取，失敗則進行建立
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
// 此時根據對象工廠，建立bean，建立時檢測依賴情況
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton()
// 進行 singleton 建立時，做安全檢查，成功後放入緩存中
/**
* Return the (raw) singleton object registered under the given name,
* creating and registering a new one if none registered yet.
* @param beanName the name of the bean
* @param singletonFactory the ObjectFactory to lazily create the singleton
* with, if necessary
* @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 建立前檢查，循環依賴失敗在此時檢測到，即單例隻能被建立一次
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// createBean()
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 最後，清除正在建立的辨別
afterSingletonCreation(beanName);
}
// 單例建立成功，則将其加入到緩存中，并清理之前的标記
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
/**
* Add the given singleton object to the singleton cache of this factory.
* <p>To be called for eager registration of singletons.
* @param beanName the name of the bean
* @param singletonObject the singleton object
*/
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
} 如上代碼，說明了bean的建立過程，我們主要看 singleton。
1. 嘗試從緩存中擷取bean，可能是提前暴露的，也可能是已經完全初始化好的；（提前暴露解決循環依賴）
2. 如果擷取不到，則進行一次完整的初始化，通過 對象工廠的 getObject() 進行對象建立；
3. 建立前，先将正在建立中辨別寫入，進而防止後續又有進行建立動作；（抛出建立異常）
4. 建立好 singleton 後，将多餘辨別删除，并将 singleton 放入緩存，以待下次直接使用；
最後，我們用一個示意圖描述下整個過程：
思考題：
依賴注入使代碼更簡單和靈活，可以自動查找關聯關系！當發現有依賴時，相當于遞歸生成裡層bean執行個體！而每個bean的建立都經過n層方法的調用；
那麼請問，在建立bean時有沒有可能導緻棧溢出？？
不要害怕今日的苦，你要相信明天，更苦！